Various aluminum parts are cast by pouring molten aluminum or molten aluminum alloy (hereinafter referred to simply as “aluminum”) into cavities in casting molds.
In the process of casting aluminum parts, an oxide film tends to be formed on the surface of molten aluminum that is poured into the mold cavities. The oxide film thus formed increases the surface tension of the molten aluminum and lowers the flowability of the molten aluminum, causing a variety of casting defects.
There have been known techniques for preventing the above shortcomings as disclosed in Japanese laid-open patent publications Nos. 2001-321916, 2001-321919, and 2001-321920, for example. Specifically, as shown in FIG. 10 of the accompanying drawings, a mold 1 has a cavity 1a for receiving molten aluminum 3 poured from a molten metal tank 2 through a hole 4 in the mold 1. The cavity 1a in the mold 1 is connected to a nitrogen gas container 6 by a pipe 5a, and also connected to a vacuum generating device (not shown) by a reduced-pressure pipe 5b (see Japanese laid-open patent publication No. 2001-321919).
An argon gas container 7 is connected to a heating furnace (metal gas generating device) 9 by a pipe 8. The argon gas container 7 is also connected by a pipe 10 to a tank 11 containing a magnesium powder, which is connected to the pipe 8 by a pipe 12.
The heating furnace 9 has an interior space that can be heated to a predetermined temperature by a heater 13. The heating furnace 9 communicates with the cavity 1a through a pipe 14 and a pipe 15. The heating furnace 9 incorporates therein a restricting means (not shown) for preventing magnesium from being delivered in a powder form into the pipe 14.
The system shown in FIG. 10 operates as follows: A nitrogen gas is introduced from the nitrogen gas container 6 through the pipe 5 into the cavity 1a in the mold 1, purging air from the cavity 1a. Therefore, a substantially oxygen-free atmosphere is developed in the cavity 1a. An argon gas is introduced from the argon gas container 7 through the pipe 8 into the heating furnace 9, from which oxygen is removed.
Then, an argon gas is introduced from the argon gas container 7 through the pipe 10 into the tank 11, delivering the magnesium powder from the tank 11 through the pipes 12, 8 into the heating furnace 9. The interior of the heating furnace 9 has been heated by the heater 13 to a temperature equal to or higher than the temperature at which a magnesium powder sublimes. Therefore, the magnesium powder supplied to the heating furnace 9 sublimes into a magnesium gas, which is introduced through the pipes 14, 15 into the cavity 1a. The cavity 1a is also supplied with the nitrogen gas from the nitrogen gas container 6, as described above.
In the cavity 1a, the magnesium gas and the nitrogen gas react with each other, generating magnesium nitride (Mg3N2). The magnesium nitride is precipitated as a powder on the inner wall surface of the cavity 1a. Preferably, the pressure in the cavity 1a is lowered by the vacuum generating device (not shown) to attract the magnesium nitride to the inner wall surface of the cavity 1a. 
Then, the molten aluminum 3 in the molten metal tank 2 is poured through the hole 4 into the cavity 1a. Since the magnesium nitride is a reducing substance (active substance), when the molten aluminum 3 is brought into contact with the magnesium nitride in the cavity 1a, oxygen is removed from the oxide film on the surface of the molten aluminum 3. Therefore, the surface of the molten aluminum 3 is reduced to pure aluminum.
The conventional system shown in FIG. 10 is disadvantageous in that the system is considerably large in overall size because it has the heating furnace 9 combined with the heater 13. Therefore, the amount of heat required to cause a reaction between the magnesium gas and the nitrogen gas is large. The pipe 14 for introducing the magnesium gas produced in the heating furnace 9 into the cavity 1a is relatively long. Furthermore, the pipes 5, 14, 15 are connected to the mold 1. For these reasons, when the mold 1 is to be replaced, many replacing steps are involved and the entire replacement process is complex. It is difficult to control the reaction of the magnesium powder in the heating furnace 9, and the substance (magnesium) produced by the reaction is deposited in the heating furnace 9.
The vacuum generating device (not shown) used to develop an oxygen-free environment in the cavity 1a also makes the overall system considerably large in size. In addition, the need for a sealing structure for hermetically sealing the cavity 1a makes the system complex.
Japanese laid-open patent publications Nos. 2001-321918 discloses a method of casting aluminum. Specifically, as shown in FIG. 11 of the accompanying drawings, a mold 1 has a cavity 1a for receiving molten aluminum 3a poured from a molten metal tank 2a through a hole 4a in the mold 1. The cavity 1a in the mold 1 is connected to a nitrogen gas container 6a by a pipe 5. An argon gas container 7a is connected to a heating furnace 9a by a pipe 8a. 
The argon gas container 7a is also connected by a pipe 10a to a tank 16 containing a magnesium powder. The tank 16 is connected to a metered quantity storage unit 18 which is connected to the pipe 8a. The heating furnace 9a communicates with the cavity 1a through a pipe 14a. A pressure-reducing pump 19 is connected to the mold 1 for reducing the pressure in the cavity 1a. 
Operation of the system shown in FIG. 11 will be described below. The interior of the heating furnace 9a is heated by the heater 13 to a temperature equal to or higher than the temperature at which a magnesium powder sublimes. Thereafter, an argon gas is introduced from the argon gas container 7a through the pipe 8a and the heating furnace 9a into the cavity 1a in the mold 1, purging air from the cavity 1a. 
Then, an argon gas is introduced from the argon gas container 7a through the pipe 10a into the tank 16, delivering the magnesium powder from the tank 16 into the metered quantity storage unit 18. The metered quantity storage unit 18 then supplies a metered amount of magnesium powder through the pipe 8a into the heating furnace 9a. The magnesium powder delivered into the heating furnace 9a sublimes into a magnesium gas, which is carried by the argon gas into the cavity 1a. 
At this time, the pressure-reducing pump 19 is actuated to replace the existing gas in the cavity 1a with the magnesium gas and the argon gas, so that the magnesium gas is diffused in the cavity 1a. Then, a nitrogen gas is introduced from the nitrogen gas container 6a through the pipe 5 into the cavity 1a. In the cavity 1a, the magnesium gas and the nitrogen gas react with each other, generating magnesium nitride (Mg3N2). The magnesium nitride is precipitated as a powder on the inner wall surface of the cavity 1a. 
Then, the molten aluminum 3a in the molten metal tank 2a is poured through the hole 4a into the cavity 1a. Since the magnesium nitride is a reducing substance, when the molten aluminum 3a is brought into contact with the magnesium nitride in the cavity 1a, oxygen is removed from the oxide film on the surface of the molten aluminum 3a. Therefore, the surface of the molten aluminum 3a is reduced to pure aluminum.
The conventional system shown in FIG. 11 is problematic in that the system is considerably large in overall size because it has the heating furnace 9a. In addition, it is difficult to control the reaction between the magnesium gas and the nitrogen gas in the cavity 1a, with the result that the amount of magnesium nitride produced in the cavity 1a is not sufficient, for example.